Frugivorous Southern Muriquis fall back on leaves when fruits are unavailable. How do they decide what leaves to eat? Color and toughness are important, but while color is salient at long-distance, perception of toughness requires biting. This leaves a substantial sensory gap in efficient foraging for the most nutritious leaf . A possible intermediate characteristic is whether a leaf supports itself against gravity. Inability to do so (floppiness) is visible at short distance. Although floppiness can result from turgor loss (wilting), it is universal in flushing leaves because they lack a venation capable of self-support. Here we hypothesize that (a) leaves pass through a floppy membrane-to-stiff plate transition as they mature and (b), muriquis prefer membranes. Mechanical and color data were collected for leaves of two tree species (Nectandra oppositifolia, Lauraceae and Guapira opposita, Nyctaginaceae). Prior to testsing, each leaf was marked for whether muriquis consume that leaf stage or not. A critical value of λ, calculated from blunt indentation, differentiated membranes (λ > 350) from plates (λ < 350). Results show turgid leaves of both species decreasing from λ= ~8000 to ~30 during development. Significant correlations with both colour and toughness support its potential role in leaf selection. Muriquis appeared to select membranous leaves: for N. oppositifolia, mean λ = 2060.1; Mann-Whitney U = 113; p < 0.012; n = 45; for G. opposita, mean λ = 437.7; U = 166.0; p < 0.009; n = 49. These findings emphasize the importance of mechanical properties in leaf foraging.